An optical device, i.e., an optical scanner according to a prior art is disclosed in Japanese Patent Application Publication No. H11-52278. The scanner 100 has a construction shown in FIGS. 37A and 37B. The scanner 100 includes a silicon substrate 110 having a concavity 101. A movable portion 103 is formed on an opening of the concavity 101. The movable portion 103 is supported by a pair of torsion bars (i.e., torsion beams) 102a, 102b. A mirror 104 is formed on the movable portion 103. A pair of fixed electrodes 106a, 106b is formed on the silicon substrate 110 through an insulation film 105.
Each fixed electrode 106a, 106b faces a movable electrode 107a, 107b of the movable portion 103, respectively. Electric voltage is applied alternately between the fixed electrode 106a, 106b and the movable electrode 107a, 107b, so that the movable portion 103 is oscillated with an electro-static force generated between the fixed electrode 106a, 106b and the movable electrode 107a, 107b. At this time, the movable electrode 107a, 107b does not interfere with the fixed electrode 106a, 106b, for example, the movable electrode 107a, 107b does not touch with fixed electrode 106a, 106b. 
To broad a scanning angle of the scanner 100, it is required to enlarge the maximum oscillation angel of the movable portion 103. In view of this requirement, for example, the applied voltage between the fixed electrode 106a, 106b and the movable electrode 107a, 107b is increased, or a diameter of the torsion bar 102a, 102b is decreased so that a torsional rigidity of the torsion bar 102a, 102b is reduced. However, in case of increasing the applied voltage, a power supply for supplying the applied voltage is required to increase its voltage. Therefore, electric power consumption is increased. In case of decreasing the torsional rigidity, the torsion bar 102a, 102b may be broken with fatigue failure, so that durability of the torsion bar 102a, 102b is reduced (i.e., a fatigue strength of the torsion bar 102a, 102b is reduced).